I have only done about 10 or so butts on my smoker, so I wont pretend to be a pro. But my best results by far, when it came to using rub, ( I was having problems with getting a good seasoned flavor throughout the butt) using the mustard definitely helped. But I have done several butts that have been brined, and some that have not been brined, and the ones that were brined came out much more tender, juicey, and had a better flavor overall than those that weren't.
So now before I do a pork butt, I brine for at least 8 hours, take it out, pat it good and dry, ( I think paper towels are a BBQ-er's best friend) then apply my mustard, and shake the rub on every possible surface, rub it in good, and then sprinkle even more as a dusting. I'm pretty happy with the results.